1. Field of the Invention
The present invention concerns friction clutches with low disengagement force, notably for motor vehicles, and relates more particularly to the engagement mechanism in such a clutch.
2. Description of Related Art
As is known, in a conventional clutch interposed between a driving shaft and a driven shaft, a diaphragm bears on the base of a cover, fixed to a flywheel for driving in rotation, in order to move a thrust plate in the direction of the flywheel, forming a reaction plate, in order to clamp the friction linings of a clutch friction device between the thrust and reaction plates.
The driving flywheel is fixed with respect to rotation to a first shaft, such as a driving shaft, whilst the clutch friction device has at its internal periphery a hub for fixing it with respect to rotation to a second shaft, such as a driven shaft.
The clutch is therefore normally engaged, with torque transmission between the driving and driven shafts.
In order to disengage, or declutch, the clutch, it is necessary, using a clutch release bearing, to act axially by pushing, in the case of a clutch of the pushed type, on the internal end of the diaphragm fingers in order to pivot the diaphragm and cancel out the force exerted by this diaphragm on the axially movable thrust plate in order to release the friction linings. The torque is then no longer transmitted from the driving shaft to the driven shaft, since the friction linings are no longer clamped between the thrust and reaction plates fixed with respect to rotation to the hollow-shaped cover.
Normally, the cover, thrust plate and diaphragm form a unitary assembly referred to as the clutch mechanism, which is attached by its cover to the flywheel, elastic tongues rotatably connecting, with axial mobility, the thrust plate to the cover.
The diaphragm, as is known, has a central hole and an external peripheral part in the form of a Belleville washer extended inwards by a central part divided into radial fingers by slots. This diaphragm, through its Belleville washer part, bears on the cover and on the thrust plate. Thus the disengagement device of the clutch, which makes it possible to counteract at will the action of the elastic engagement means, consists of the fingers of the diaphragm on the internal end of which the clutch release bearing acts, whilst the Belleville washer of the diaphragm constitutes elastic axially acting engagement means for clamping the friction linings between the thrust and reaction plates and therefore to axially force the thrust plate in the opposite direction to the base of the cover.
In the free state, this diaphragm has a frustoconical shape. Once mounted in the clutch, its Belleville washer is mounted under prestress and is more or less flattened. During the operation of declutching, or disengaging, the clutch, the conicity of its Belleville washer is modified.
As is known, the characteristic curve of this diaphragm, which represents the force exerted according to the disengagement travel, for example returned to the level of the inner end of the diaphragm fingers, is determined by the dimensions of its Belleville washer, notably by the ratio between the height of the truncated cone of the Belleville washer in the free state and the thickness of the diaphragm.
This characteristic curve passes through a maximum.
Thus the force to be exerted on the internal end of the diaphragm fingers, during the disengagement operation, increases up to a maximum, decreases gradually to a minimum, and then increases once again.
The difference between the maximum and the minimum can be great. For more information on this characteristic curve, reference should be made for example to the document FR-A-1 392 569 or to the document WO-97/19 275.
In the document FR-A-1 392 569, provision is made for an elastic progressive-action device for avoiding passing through the aforementioned maximum during the disengagement travel. This device is mounted in series with the Belleville washer of the diaphragm and has an elastic force which is appreciably less than that of the Belleville washer. This device has a limited travel between a prestressing position where its force is at a maximum and a stressing position where its force is at a minimum.
With this arrangement an overall increasing disengagement force is obtained at the clutch release bearing.
It is not possible to obtain, during the disengagement travel, an assistance which is as great as desired, the progressive-action device reducing the load exerted by the diaphragm on the thrust plate during this engagement.
The load curves of the progressive-action device and of the diaphragm have shapes which do not make it possible to combine them in order to obtain strong assistance with disengagement, particularly if it is wished to comply with the disengagement travels and forces compatible with the actual disengagement commands. Moreover, the shape of the curve of the progressive-action device changes very rapidly with the life of the clutch, and the higher the assistance level required, the more this cancels out the disengagement assistance. In order to prevent aging of the curve of the assistance device, it is preferably to place this at a point where its stiffness curve will remain stable over time and, in particular, at a place positioned outside the progressive-action area of the friction device where the lining contact and the phenomenon of incrustation change and cause the progressive-action curve to change.
Thought can then be given to causing an assistance spring to act in parallel with the diaphragm. In this case, the assistance spring, for example in the form of a Belleville washer, can bear on the cover and on the inner end of the diaphragm fingers.
In the clutch engaged position, it can then exert a minimum force, and then, its conicity varying, exert an assistance force during the disengagement operation. It is therefore necessary for this assistance force to be at a minimum in the clutch-engaged position throughout the life of the clutch.
The idea for this type of design is to have recourse to an adjustment device, referred to as a wear take-up device, which always holds the diaphragm roughly in the same position, whatever the wear on the friction linings of the clutch and/or the friction faces of the thrust and reaction plates, so that the assistance spring exerts, in all circumstances during the life of the clutch, a very low force when the clutch is in the engaged position.
In the certificate of addition 86 983 to French patent 1 392 569, it has already been proposed to interpose, between the elastic washer of the diaphragm and the thrust plate, an auxiliary elastic washer, the two elastic washers being placed in series. The auxiliary elastic washer affords a certain degree of progressive action of the engagement of the clutch and procures a certain amount of assistance force during its disengagement.